border-1px($color)  // 定义函数,出入一个颜色
  position: relative
  &:after
    content: ' '
    display: block
    position: absolute
    left: 0
    bottom: 0
    width: 100%   // 绝对定位，要设置相对父元素100%
    border-top: 1px solid $color


bg-image($url)
    background-image:url($url+'@2x.png')
@media (-webkit-min-device-pixel-ratio:3),(min-device-pixel-ratio:3)
    background-image:url($url+'@3x.png')
